Breast cancer, the leading cancer in women, is a disease for which the established risk factors still explain only a small fraction of the population attributable risk. One of the more provocative new etiologic candidate hypotheses is that of increased risk associated with exposure to electromagnetic fields (EMF), an exposure both ubiquitous in the environment and a current focus of public concern. This proposal seeks to take advantage of a unique opportunity to build a rigorous case-control study from an evaluation of breast cancer incidence in California teachers, a group shown to experience higher rates of breast cancer and for which public concern in recent years has focused on EMF associated risks. California has a work-force of about 158,000 female school teachers representing to varying degrees the unusual racial/ethnic and cultural diversity of the state. Information about California's teachers is maintained yearly by the Department of Education in a computerized database since 1981. Also, California has a statewide population-based cancer surveillance system, established in 1988. Public utilities in the state have or will provide the Department of Health Services with a database of the types of power lines and electrical facilities near schools. These three data sources will provide the foundation for the proposed study. The proposed study will link the teachers cohort with the utility school survey and the statewide cancer incidence files to establish a study cohort with outcome information, and draw from this study cohort a nested case-control sample to enroll in a epidemiologic interview and EMF measurement study. This study will take a multistage approach to the problem by: 1. Examining preliminary associations in a large cohort, all California teachers, 2. Re-evaluating those risk associations using more detailed measures of exposure adjusting for known risk factors for breast cancer in a nested case-control interview study drawn from the teachers cohort, and 3. Further refining the risk relationships associated with measured EMF exposure profiles in a second stage sample of the interviewed teachers. The proposed study offers a unique opportunity to rigorously evaluate the EMF-risk hypothesis in a well characterized sample of women thought to be at elevated risk for breast cancer. The availability of valuable pre-existing statewide data files for the cohort, for school-related EMF powerline frequency exposures, and for breast cancer outcomes significantly strengthens the ability to conduct such a study at a minimum of cost. The findings from this study will make a major contribution to both scientific and public health concerns about EMF.